Noise suppression



April 30, 1940. H H RE 2,199,190

NOISE SUPFRESSION Filed Sept. 29, 1936 Fig.1

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. 22 X i 4 y NON LINEAR 5 1 Z RES/STANCE Z0 :9- -62 W 1 N 1 x w Q 5INVENTOR HENRY SHORE ATTORNEY Patented Apr. 30, 1940 UNITED STATESPATENT OFFICE NOISE SUPPRESSION ware Application September 29, 1936,Serial No. 103,061

2 Claims.

This invention relates to noise suppression which is based on the actionof a non-linear resistance in series with a load circuit.

Heretofore, noise suppressors have been de- 5 vised which require rathercomplicated circuits and make use of vacuum tubes in addition to thesaid complicated circuits. This invention makes use of a non-linearresistance element, acting as a passive resistance in series with theload and thereby overcomes maintenance costs, high initial costs and hasthe advantage of being exceedingly simple. Any non-linear resistanceknown may be used provided it has the necessary characteristics. Thecharacteristics of the non-linear resistance used in my novel circuitmust be such that increasing the voltage applied to the resistance willdecrease its resistance at a rapid rate. This decrease in resistancecauses an increase in the current flow through the resistance. If thenon-linear resistance is placed in series with a load resistance it willuse up a relatively large amount of small impressed potentials and smallamount of large impressed potentials. These characteristics may be usedin accordance with my novel concept to reduce the noise heretoforepresent in the outputs of potential or wave amplifiers of all types.

As pointed out above, any non-linear resistance having the propercharacteristics may be 80 used. One substance particularly adapted tothe present invention is that known in the art as Thyrite. Thecharacteristics of Thyrite have been described in detail by Mr. K. B.McEacheron in the General Electric Review of February, 1930, volume 33,No. 2, page 92. For a detailed description of this type of non-linearresistance reference is made for those interested, to said publication.The character of Thyrite is such that doubling the voltage applied to itwill decrease its resistance to less than one-sixth its initial value.This decrease in resistance is obviously accomplished by a rapidincrease in current flow through the Thyrite. Another characteristic ofThyrite is that it conducts currents equally well in either direction ofthe conductive path through its resistive material. A Thyrite resistormay, therefore, be regarded as unbiased with respect to the polarity ofthe currents traversing the same. The composition of Thyrite and theprocess of making it is of no consequence in the disclosure or myinvention since this resistive material is one which is commerciallyobtainable for a great many different purposes. It may be said, however,that Thyrite has mechanical properties somewhat similar to those ofdry-process porcelain, but possesses the remarkable property of beingsubstantially an insulator atone potential and a good conductor atcertain higher potentials. The process of making Thyrite is said toinvolve the very accurate control of a large number of variables. Thematerial can be moulded in any desired shape and its consistency can bedepended upon as being held within close limits so that, for example, adisc of a given thickness may be depended upon as having a predeterminedresistance value in relation to the applied voltage.

When considering the application of Thyrite it should be noted that theconstant varies directly as the thickness of the piece but not inverselywith the area as does the ordinary resistance. For instance, with anexponent of one, the constant for the piece depends on the length but isindependent of the area. To make this clear, we may consider the effectof doubling the area, remembering that the constant is defined as theresistance at one ampere, which, with an exponent of one, is inverselyproportional to the current. By doubling the area, the current in theoriginal area is reduced to one-half, but the resistance of each half isthereby doubled and so the total resistance remains unchanged. It shouldbe noted that these relationships are dependent upon the definition ofthe constant as the resistance at one ampere. As the exponent approacheszero as a limit these relationships approach those of the ordinaryresistor.

Although my invention has been described clearly to those skilled in theart, additional description thereof be made and in said additionaldescription reference will be made to the drawing attached hereto.

In the drawing,

Figure 1 shows curves illustrating the volt amperes and volt ohms,characteristics of a nonlinear resistance such as Thyrite;

Figure 2 illustrates somewhat diagrammatically the essential elements ofan amplifying or relaying system including the noise suppressing featureof the present invention;

Figure 3 shows curves illustrating the manner in which the non-linearresistance acts in the relaying or amplifying circuit to suppress,reduce, or eliminate the noise so that a more favorable signal to noiseratio is obtained in the energy reaching the load circuit as has beenthe case heretofore; while Figure 4 is a simplified diagram used inillustrating values of resistances necessary to obtain a specified noisereduction;

Referring to the drawing and in particular to Figure 1, it will be notedthat the resistance value of the non-linear resistor decreases rapidlywith an increase in voltage applied to the terminals thereof. Thisdecrease in resistance value is accompanied by a rapid increase incurrent which may flow through the non-linear resistor. The curves ofFig. 1 hold true both for a rising voltage and a falling voltage, thusindicating that there is no appreciable time lag in the resistivecharacteristic of the preferred non-linear resistor which I haveadopted, namely, 'I'hyrite. The curves of Figure 1 are characteristic ofa low resistance piece of non-linear resistance. Increasing the lengthof the non-linear resistance increases the resistance as does decreasingthe diameter.

In Figure 2 I have shown a source of wave energy or potentials ormodulated wave energy in the form of an aerial system 4 connected withthe input electrodes of a thermionic amplifier 6 having its outputelectrodes coupled to a rectifier 8, in turn coupled by a low frequencytransformer ID to a load circuit 20. A non-linear resistance 22 isconnected as shown in series with the load circuit. The load circuit maybe any utilization device or additional amplifiers but is preferably afinal utilization device such as a recorder or a loudspeaker, etc. Thesource 4 may be replaced by incoming lines or the source 4 and theamplifier 6 and the demodulator 8 may be replaced by any source of lowfrequency potentials or wave energy or modulated wave energy. Bymodulation it is intended to include keyed wave energy or tone keyedenergy.

In Figure 2 it will be noted that the non-linear resistance element 22,preferably composed of Thyrite, is placed in series with the load 20.The load in this case is simply the device which is to work from anincoming signal. It may, for example, be a relay or an amplifier or aloudspeaker. Due to the non-linear characteristics of the non-lineardevice 22 decreasing current through it increases its resistance very,very rapidly. Consequently, when a fixed value of resistance such as aload is placed in series with the non-linear resistance, as the incomingwave signal voltage decreases, a greater and greater portion of theavailable voltage is used up across the non-linear resistance. In otherwords, the voltage drops across the non-linear resistance for very lowapplied voltage values constitutes by far the greater portion of thetotal drop in the circuit including the load. This means that for lowapplied voltages very little of the applied voltage is impressed acrossthe load. As the applied voltage, which may carry a signal, increaseshowever, more and more of said voltage appears across the load. Thisbeing so, since the noise level of the amplifying or relaying system isgenerally much lower than the useful voltage or signal, the non-linearresistance has a tendency to suppress the noise voltage coming inwithout materially attenuating the received useful Voltage. This effectof the circuit is illus trated graphically in Figure 3 wherein thevoltage across a load is plotted against the incoming applied voltage orsignal, the straight line 30 denotes the relation between the voltageacross the load and the applied voltage when no nonlinear resistance isused in series with the load circuit. The curved line 32 denotes therelation between the voltage applied across the load and the incomingvoltage or signal when a nonlinear resistance is, in accordance with myinvention, placed in series with the load. As will be seen, little or novoltage resulting from noise, reaches the load circuit in the lattercase.

More specifically, the reduction of noise ratio by the use of my novelnetwork is obtained in practice as follows:

Assume that wave energy or varying potentials are applied to theterminals X of the network of Figure 4 and it is desired to obtainpotentials or wave energy of improved signal to noise ratio from theterminals Y. For purposes of illustration, also assume that thenon-linear resistance has a value of 10,000 ohms so that when onemilliampere current is passed through said non-linear resistance a dropof potential of 10 volts will be produced therein. This one milli-ampereof current is also flowing through the substantially linear resistance20 and, if this resistance is given a value of 10,000 ohms, it willproduce a drop of 10 volts. Thus, in an example given we will see thatthe voltage impressed across the terminals X is equal to the sum of thepotential drops in 22 and 20, which is 20 volts. With a one volt dropacross the non-linear resistance 22 the resistance is equal tosubstantially 60,000 ohms (due to the non-linear characteristic of saidresistance) and consequently a current of .0167 milliampere flows in thesaid non-linear resistance. This means that across the series outputresistance there is produced a drop of 1.67 volts. This means that thetotal voltage impressed across the terminals X is the sum of thepotential drops in 22 and 20 or 1.167 volts.

Thus, if the 20 volts impressed represented signal voltage and the 1.167volts represents the noise, the signal to noise ratio impressed on theinput terminals X is about 17 to 1. On the other hand, the ratio ofoutput voltages at the terminals Y is 10 to .167 or a signal to noiseratio of about 60 to 1. Hence my network insures considerableimprovement of increase of the signal to noise ratio in signals passedthereby.

In usual signalling practice for radio the minimum output signal tonoise ratio in practice is considered to be about 20 to 1. Obviously,the arrangement of the present invention enables me to use a signalratio (17 to 1, and less) heretofore considered not usable.

I claim:

1. In a wave amplifying system, means for receiving and amplifying waveenergy, a load circuit, a resistor in series with said load circuit,said resistor being constituted by a substance having bilateralconductivity and a non-linear voltage current characteristic, and meanscoupling said load circuit and non-linear resistance to said amplifyingmeans.

2. A radio receiving circuit having an antenna, a detector, anamplifier, and means for attenuating the noise component of signallingenergy impressed upon said circuit, said means comprising a singleresistor whose conductivity is inde pendent of the direction of currentflow therethrough, and whose impedance value is an inverse logarithmicfunction of the voltage applied thereacross, and a utilization circuitof substantially linear impedance characteristics to which saidsignalling energy is fed through said attenuating means.

HENRY SHORE.

